The nature of photovoltaic cells, terminology employed and the general background of the present invention have been described in the background section of U.S. Pat. No. 4,239,553, relating to thin film photovoltaic cells wherein one of the present co-inventors is also a co-inventor in that patent, which is incorporated herein by reference.
That patent points out that thin film photovoltaic cells have many potential advantages. Of the four types of thin film cells which in the laboratory have demonstrated efficiencies greater that 10 percent, three are based on polycrystalline thin film semiconductors. One of the semiconductors in each of these polycrystalline cases is chosen from elements of groups II and VI of the periodic system of elements, in particular, cadmium sulfide (CdS) or zinc cadmium sulfide (ZnCd)S. The ternary compound, (ZnCd)S, with its additional component, provides additional opportunities for higher energy conversion efficiency without introducing expensive raw materials.
The utility of (ZnCd)S in photovoltaic conversion resides in this ability to engineer certain of its properties to optimize photovoltaic conversion efficiency. Specifically, these properties include optical band gap, electron affinity, majority carrier concentration and lattice constant. These properties are acutely sensitive to changes in the composition of (ZnCd)S.
The empirical formula Zn.sub.x Cd.sub.1-x S, where x is a positive number less than one, is indicative of the variability of this composition. As might be expected of this three-component compound having a wide latitude in the content of the metal elements, difficulty could be experienced during production in achieving and maintaining the desired ratio of these elements upon which the properties are dependent. Primary consideration must be given in the choice of a process to its suitability for precise control and uniformity of the composition produced.
Several techniques are known in the art for formation of Zn.sub.x Cd.sub.1-x S thin films on substrates:
(a) Spray pyrolysis: PA1 (b) Chemical vapor deposition; PA1 (c) Sputtering, including reactive and compound targets; PA1 (d) Co-evaporation of CdS and ZnS; and PA1 (e) Elemental evaporation. PA1 (a) it must provide an adherent coating precise and uniform in composition, crystallographic structure and thickness; PA1 (b) it must be capable of operation at high production rates (high rates of deposition of thin film on substrate); and PA1 (c) it must effectively utilize materials, particularly with little wastage of costly components.
Further elaboration of the details of each method is unnecessary at present; it is important to observe, however, that the method selected for commercial use must meet certain requirements:
In the elemental evaporation method the component elements of Zn.sub.x Cd.sub.1-x S, (along with any dopant, as appropriate), are simultaneously evaporated from heated vessels in a vacuum and allowed to impinge on a surface of a substrate where they chemically combine to form the desired composition.
In spite of the extent to which technology has been directed to developing Zn.sub.x Cd.sub.1-x S semiconductor films for photovoltaic applications and the like, the prior art has not provided a solution to the problem of composition control, raw materials utilization and uniformity in the product in a process suitable for commercial use.
The present invention is addressed to the objects of providing a process of forming and depositing Zn.sub.x Cd.sub.1-x S coatings of precise composition with high, efficient utilization of metal.